Recently, techniques for acquiring higher capacity by reducing the quantity of frequency resources required to exchange signals between terminals using network coding in a bidirectional relay system have been proposed.
FIG. 1 is a diagram illustrating point-to-point network coding according general technology.
FIG. 1(a) illustrates an example of general bidirectional communication that does not use network coding. From this drawing, it can be seen that when a first terminal U1 and a second terminal U2 desire to exchange packets x1 and x2 using a relay R, a total of four orthogonal frequency resources are required in order to avoid interference.
FIG. 1(b) illustrates an example in which digital network coding is used in a bidirectional relay system. In this example, a first terminal U1 and a second terminal U2 transmit packets x1 and x2 to a relay R over two different orthogonal channels, and the relay R decodes the signals received from the first terminal U1 and the second terminal U2, performs an XOR operation on the decoded signals, and then broadcasts a resulting signal.
The first terminal U1 extracts desired packet x2 by performing, again, the XOR-operated signal received from the relay R and the packet x1 transmitted by itself and stored in memory.
In this case, the bidirectional relay system uses only a total of three orthogonal resources, and thus high capacity can be acquired by increasing frequency efficiency.
FIG. 1(c) illustrates a communication technique based on an analog network coding method that is an improvement over the method illustrated in FIG. 1(b). In this technique, a first terminal U1 and a second terminal U2 perform simultaneous transmission to a relay R via the same channel using only a total of two orthogonal resources, and the relay R only amplifies a signal received in a superposed manner and then retransmits the amplified signal.
In this case, the first terminal U1 obtains desired packet x2 by eliminating the signal x1 transmitted by itself from the superposed signal received from the relay R and then performing decoding.
Such an analog network coding method has the advantage of being easily implemented because it is simpler than a digital network coding method and the advantage of achieving higher capacity in a high signal-to-noise (SNR) range than the digital network coding method because it uses fewer frequency resources.
However, in a conventional analog network coding method, since feeders, for example, the first terminal U1 and second terminal U2 of FIG. 1, that directly transmit data to a satellite use directional antennas in order to increase transmission and reception power efficiency, the conventional analog network coding method is problematic in that it can perform only point-to-point bidirectional communication and it is difficult to extend the conventional analog network coding method to many-to-many multicast-to-multicast bidirectional communication.
That is, in a satellite communication system using the conventional analog network coding method, reception nodes adjacent to a feeder cannot obtain transmission data transmitted to a satellite from the feeder, and thus it is impossible to obtain desired information from a superposed signal even when the superposed signal is received from the satellite, with the result that many-to-many bidirectional communication cannot be performed.
Accordingly, there is a need for a method that is capable of performing many-to-many communication while using an analog network coding method.
Meanwhile, an analog network coding method in a satellite network is advantageous in that it is difficult for a third party to eavesdrop and communication security can be easily maintained due to the fact that only one-to-one bidirectional communication is possible. However, if the analog network coding method is extended to many-to-many communication in a satellite network, there is concern that the secrecy of communication may be divulged during the process of distributing a key used to interpret a signal.
As a result, there is a demand for a means for maintaining the secrecy of communication and also enhancing security while extending the application field of the conventional analog network coding method for point-to-point bidirectional communication to many-to-many bidirectional communication.